forcedecode: tighten checks

...and fix a few golint issues and print a scary warning message on mount.

Also, force the fs to ro,noexec.

Documentation/MANPAGE.md

@@ -67,11 +67,16 @@ failing with an IO error. Warning messages are still printed to syslog if corrup
 files are encountered.
 It can be useful to recover files from disks with bad sectors or other corrupted
 media. It shall not be used if the origin of corruption is unknown, specially
-if you want to run executable files. For corrupted media, note that you probably want
-to use dd_rescue(1) instead.
-This option has no effect in reverse mode. It requires gocryptfs to be compiled with openssl
+if you want to run executable files.
+
+For corrupted media, note that you probably want to use dd_rescue(1)
+instead, which will recover all but the corrupted 4kB block.
+
+This option makes no sense in reverse mode. It requires gocryptfs to be compiled with openssl
 support and implies -openssl true. Because of this, it is not compatible with -aessiv,
-that uses built-in Go crpyto.
+that uses built-in Go crypto.
+
+Setting this option forces the filesystem to read-only and noexec.
 
 #### -fsname string
 Override the filesystem name (first column in df -T). Can also be

cli_args.go

@@ -171,11 +171,17 @@ func parseCliOpts() (args argContainer) {
 			tlog.Fatal.Printf("The reverse mode and the -forcedecode option are not compatible")
 			os.Exit(ErrExitUsage)
 		}
-		v, e := strconv.ParseBool(opensslAuto)
-		if e == nil && v == false {
-			tlog.Warn.Printf("-openssl set to true, as it is required by -forcedecode flag")
+		// Has the user explicitely disabled openssl using "-openssl=false/0"?
+		if !args.openssl && opensslAuto != "auto" {
+			tlog.Fatal.Printf("-forcedecode requires openssl, but is disabled via command-line option")
+			os.Exit(ErrExitUsage)
 		}
 		args.openssl = true
+
+		// Try to make it harder for the user to shoot himself in the foot.
+		args.ro = true
+		args.allow_other = false
+		args.ko = "noexec"
 	}
 	// '-passfile FILE' is a shortcut for -extpass='/bin/cat -- FILE'
 	if args.passfile != "" {

internal/contentenc/content.go

@@ -86,7 +86,9 @@ func (be *ContentEnc) DecryptBlocks(ciphertext []byte, firstBlockNo uint64, file
 		var pBlock []byte
 		pBlock, err = be.DecryptBlock(cBlock, firstBlockNo, fileID)
 		if err != nil {
-			if be.forceDecode == false || (be.forceDecode == true && stupidgcm.AuthError != err) {
+			if be.forceDecode && err == stupidgcm.ErrAuth {
+				tlog.Warn.Printf("DecryptBlocks: authentication failure in block #%d, overriden by forcedecode", firstBlockNo)
+			} else {
 				break
 			}
 		}
@@ -139,11 +141,10 @@ func (be *ContentEnc) DecryptBlock(ciphertext []byte, blockNo uint64, fileID []b
 	if err != nil {
 		tlog.Warn.Printf("DecryptBlock: %s, len=%d", err.Error(), len(ciphertextOrig))
 		tlog.Debug.Println(hex.Dump(ciphertextOrig))
-		if be.forceDecode == true {
+		if be.forceDecode && err == stupidgcm.ErrAuth {
 			return plaintext, err
-		} else {
-			return nil, err
 		}
+		return nil, err
 	}
 
 	return plaintext, nil

internal/fusefrontend/file.go

@@ -18,6 +18,7 @@ import (
 
 	"github.com/rfjakob/gocryptfs/internal/contentenc"
 	"github.com/rfjakob/gocryptfs/internal/serialize_reads"
+	"github.com/rfjakob/gocryptfs/internal/stupidgcm"
 	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
@@ -200,9 +201,14 @@ func (f *file) doRead(off uint64, length uint64) ([]byte, fuse.Status) {
 	// Decrypt it
 	plaintext, err := f.contentEnc.DecryptBlocks(ciphertext, firstBlockNo, fileID)
 	if err != nil {
+		if f.fs.args.ForceDecode && err == stupidgcm.ErrAuth {
+			// We do not have the information which block was corrupt here anymore,
+			// but DecryptBlocks() has already logged it anyway.
+			tlog.Warn.Printf("ino%d: doRead off=%d len=%d: returning corrupt data due to forcedecode",
+				f.devIno.ino, off, length)
+		} else {
 			curruptBlockNo := firstBlockNo + f.contentEnc.PlainOffToBlockNo(uint64(len(plaintext)))
 			tlog.Warn.Printf("ino%d: doRead: corrupt block #%d: %v", f.devIno.ino, curruptBlockNo, err)
-		if (f.fs.args.ForceDecode == false) {
 			return nil, fuse.EIO
 		}
 	}

internal/stupidgcm/autherr.go

@@ -0,0 +1,8 @@
+package stupidgcm
+
+import (
+	"fmt"
+)
+
+// ErrAuth is returned when the message authentication fails
+var ErrAuth = fmt.Errorf("stupidgcm: message authentication failed")

internal/stupidgcm/stupidgcm.go

@@ -10,7 +10,6 @@ import "C"
 
 import (
 	"crypto/cipher"
-	"fmt"
 	"log"
 	"unsafe"
 )
@@ -30,9 +29,6 @@ type stupidGCM struct {
 	forceDecode bool
 }
 
-//authentication error
-var AuthError error = fmt.Errorf("stupidgcm: message authentication failed")
-
 var _ cipher.AEAD = &stupidGCM{}
 
 // New returns a new cipher.AEAD implementation..
@@ -193,10 +189,9 @@ func (g stupidGCM) Open(dst, iv, in, authData []byte) ([]byte, error) {
 		// The error code must always be checked by the calling function, because the decrypted buffer
 		// may contain corrupted data that we are returning in case the user forced reads
 		if g.forceDecode == true {
-			return append(dst, buf...), AuthError
-		} else {
-			return nil, AuthError
+			return append(dst, buf...), ErrAuth
 		}
+		return nil, ErrAuth
 	}
 
 	return append(dst, buf...), nil

internal/stupidgcm/without_openssl.go

@@ -14,9 +14,6 @@ const (
 	BuiltWithoutOpenssl = true
 )
 
-//authentication error - needed to compile as same varaible is exported when openssl is enable via stupidgcm.go
-var AuthError error = fmt.Errorf("stupidgcm: message authentication failed with openssl disabled!")
-
 func errExit() {
 	fmt.Fprintln(os.Stderr, "gocryptfs has been compiled without openssl support but you are still trying to use openssl")
 	os.Exit(2)

mount.go

@@ -249,6 +249,10 @@ func initFuseFrontend(key []byte, args *argContainer, confFile *configfile.ConfF
 		// Make the kernel check the file permissions for us
 		mOpts.Options = append(mOpts.Options, "default_permissions")
 	}
+	if args.forcedecode {
+		tlog.Info.Printf(tlog.ColorYellow + "THE OPTION \"-forcedecode\" IS ACTIVE. GOCRYPTFS WILL RETURN CORRUPT DATA!" +
+			tlog.ColorReset)
+	}
 	if args.nonempty {
 		mOpts.Options = append(mOpts.Options, "nonempty")
 	}